Natural gas, or in general mixtures of hydrocarbons that are volatile enough to make the mixture appear in gaseous form in room temperature, constitutes an advantageous alternative to fuel oil as the fuel of internal combustion engines. In sea-going vessels that use natural gas as fuel, the natural gas is typically stored onboard in liquid form, giving rise to the commonly used acronym LNG (Liquefied Natural Gas). Natural gas can be kept in liquid form by maintaining its temperature below a boiling point, which is approximately −162 degrees centigrade (−260 degrees Fahrenheit). Natural gas can be also stored for use as fuel by keeping it compressed to a sufficiently high pressure, in which case the acronym CNG (Compressed Natural Gas) is used. This description refers mainly to LNG because liquefying is considered more economical than compressing at the time of writing this text.
FIG. 1 illustrates schematically the architecture of a known system onboard an LNG-fuelled vessel. An LNG bunkering station 101 is located on the deck and used to fill up the system with LNG. The LNG fuel storage system comprises one or more thermally insulated gas tanks 102 for storing the LNG in liquid form, and the so-called tank room 103 where the LNG is controllably evapo-rated and its distribution to the engine(s) is arranged. Evaporation means a phase change from liquid to gaseous phase, for which reason all subsequent stages should leave the L for liquefied out of the acronym and use only NG (Natural Gas) instead.
The engine 104 or engines of the vessel are located in an engine room 105. Each engine has its respective engine-specific fuel input subsystem 106, which in the case of gaseous fuel is in some sources referred to as the GVU (Gas Valve Unit). The tank room 103 of FIG. 1 comprises two evaporators, of which the first evaporator 107 is the so-called PBU (Pressure Build-Up) evaporator used to maintain a sufficient pressure inside the gas tank 102. Internal hydrostatic pressure at the inlet of a main supply line 108 inside the gas tank 102 is the driving force that makes the LNG flow into the second evaporator 109, which is the MGE or Main Gas Evaporator from which the fuel is distributed in gaseous form towards the engines. In order to ensure that evaporated gas flows to the GVU(s) and further to the engine(s) at sufficiently high pressure, the PBU system maintains the internal pressure of the gas tank 102 at or close to a predetermined value, which is typically between 5 and 10 bars.
The engine 104 comprises one or more cooling circuits. Schematically shown in FIG. 1 is an external loop 110 of the so-called low temperature (LT) cooling circuit, which may be used for example to cool lubricating oil. The so-called LT water that circulates in the external loop 110 may have a temperature around 50 degrees centigrade when it goes through a heat exchanger 111, in which it donates heat to a mixture of glycol and water that in turn transfers heat to the evaporators 107 and 109. The glycol/water mixture circuit comprises a circulation pump 112 and an expansion tank 113. Glycol is needed in the mixture to prevent it from freezing when it comes into contact with the extremely cold LNG inlet parts of the evaporators 107 and 109.
The heat and fluid flows in the system of FIG. 1 are schematically illustrated in FIG. 2. LNG flows from the gas tank 102 into a PBU evaporation circuit 201 and an MGE evaporation circuit 202 that are located in the tank room 103. Heat originates from combustion (and from friction) in the engine 104, and gets transferred from the cooling water circuit 110 to the glycol/water mixture circuit 203, which in turn donates heat to the PBU and MGE evaporation circuits 201 and 202. Both of these produce gas, which in the case of the PBU evaporation circuit is led back to the gas tank 102 and in the case of the MGE evaporation circuit to the engine 104.
FIG. 3 is a slightly different schematic representation but illustrates essentially the same parts of the evaporation circuits as FIG. 1: a gas tank 102, a PBU evaporator 107, and an MGE evaporator 109. The pipes 301 and 302 on the right are the incoming and outgoing pipes of the glycol/water mixture circuit respectively.
Drawbacks of the prior art approach illustrated in FIGS. 1 to 3 include a relatively complicated structure, which requires a relatively long assembling time at the shipyard when a gas-fuelled sea-going vessel is built and causes relatively high manufacturing costs. Another disadvantageous characteristic is the relatively large number of pipes in which extremely cold LNG flows—an unexpected mechanical failure might allow the cold liquefied gas to flood the tank room and/or its surroundings.
In publication CA 2653643 discloses a pressure control system comprising separate conduits for supplying liquefied gas and vapor from a cryogen space defined by a cryogenic storage tank, in which a heat exchanger(s) with a source of heat for both evaporating the gas for engine and building pressure in the tank is used.
In publication US 2011/146605 discloses a liquefied natural gas system for a natural gas vehicle engine with flow driven by the engine includes dual flow paths through at least one heat exchanger, in which the heat exchanger(s) with a source of heat for both evaporating the LNG for engine and building pressure in the tank is used.